Proper nervous system function requires the production of diverse cell types in stereotyped positions in the embryonic central nervous system. The long-term objective of this application is to identify within the vertebrate spinal cord. The model system to be studies is the lumbosacral (LS) spinal cord of the avian embryo. During embryogenesis, the avian LS spinal cord acquires characteristic features, including a lateral motor column in which limb motoneuron target identity is distinctive at different axial or anteroposterior levels and a unique pattern of Hoxd-10 expression that varies with axial level. These characteristics become determined at early neural tube stages, that is, the cells of the LS region acquire the ability to proceed along a proper position-specific path of differentiation without further environmental control. A central aim is to identify the source of signals that govern this event. In vivo experiments will address the questions of whether inductive signals are transmitted in a planar manner from posterior to anterior neural regions and whether the remnants of Hensen's node and the primitive streak (the tailbud) are a source of signals. The cellular contributions of the tailbud to anterior LS regions will be mapped. Proposed studies will also assess the signaling functions of paraxial mesoderm and presumptive limb tissue. Techniques to be used include in vivo surgical manipulation, quail/chick chimera construction, in situ hybridization with RNA probes, and retrograde cell tracing. While regional differences may exist within the early neural tube, it is not known if motoneuron precursors are actually specified with respect to target identity at that time. Quail/chick chimeras and/or chimeras of retrovirally-infected and resistant chick strains will be used to examine the role of floorplate signals and neighbor/neighbor interactions in the specification of target identity after early neural tube stages. Overall, results will proved basic information about how neuronal diversity is obtained and may improve the design of surgical cell replacement therapies.